There is a lack of information on the effect of swine caecal microbiota on fumonisin metabolism. In this in vitro study, the biotransformation of fumonisin B(1) (FB(1)) by the gut microbiota of adult, healthy pigs was examined. Suspensions of caecal contents and McDougall buffer solution were incubated anaerobically with pure FB(1) for 0, 12, 24, 48 and 72 h. After 48 h, the conversion of FB(1) to partially hydrolysed FB(1) (46%) was nearly equal to the percentage ratio of FB(1), while by 72 h it was 49%. In vitro, the conversion of fumonisin B(1) to aminopentol was less than 1%. The results show that the caecal microbiota are capable of transforming fumonisin B(1) to the above metabolites. Further studies on FB(1) metabolism in the small intestine are clearly justified.